Over the past few years, there has been an increase in interest in renewable and sustainable energy resources which has spurred research and development in many energy resources. In particular, substantial research and development has centered around photovoltaic energy and its generation. Generally, conventional photovoltaic cells where first developed in 1958 by Bell Laboratories where a diffused silicon p-n junction was used. While the efficiencies of these conventional diffused silicon p-n junction devices improved over the decades, the best conventional diffused silicon p-n junction devices have not exceeded twenty-three percent (23%). While certainly there was improvement of the efficiency of these conventional diffused silicon p-n junction devices over the years, it has become apparent that there are fundamental limitations to the efficiency that can be achieved by diffused silicon p-n junction devices. Also, the cost of silicon wafers used for making conventional photovoltaic cells has remained high making the cost of conventional photovoltaic cells non-competitive when compared to alternative technologies.
The introduction of conventional multi-junction photovoltaic devices around 1976, showed significantly better efficiencies which improved over the decades and have achieved efficiencies ranging up to and about forty-three (43.0 percent). While this is significantly better then the conventional diffused silicon p-n junction devices, the multi-junction photovoltaic devices are still not efficient enough for some applications. More importantly, since the efficiencies of both the conventional diffused silicon p-n junction devices and the conventional multi-junction photovoltaic devices have reached a plateau for at least the last decade and for the foreseeable future. There is very little reason to believe that significantly greater efficiency can be wrung out of the conventional devices.
Both the conventional diffused silicon p-n junction devices and the multi-junction photovoltaic devices share some common problems. While in some cases each type of device has some particular problems of its own. One problem or disadvantage is that both types of conventional devices are built as discrete devices. For example, a conventional diffused silicon p-n junction device that is built having a band gap of 1.1 eV will efficiently capture light having 1.1 eV energy. Any energy of light below the 1.1 eV energy band gap is not captured. Also, any energy of light above the 1.1 eV energy band gap is wasted and not put to productive use. More specifically, a 1.1 eV band gap photovoltaic cell that absorbs a 1.2 eV photon will efficiently convert 1.1 eV of the 1.2 eV photon energy and will waste the 0.1 eV difference. Similarly, the 1.1 eV band gap photovoltaic cell that absorbs a 2.2 eV photon will efficiently convert 1.1 eV of the 2.2 eV photon energy and waste the additional 1.1 eV of photon energy. This same principle is true of any single junction device of a given energy band gap. In addition to this effect, silicon is well-known as an Indirect Band Gap material as opposed to III/V materials that are generally, but not exclusively, Direct Band Gap materials. Photovoltaic devices made with indirect band gap materials further waste energy due to their inability to completely absorb photons with energy equal to and just above their material band gap.
Conventional multiple junction devices are also discrete devices; however, because conventional multiple junction devices are typically built using III/V materials and germanium substrates, multiple junctions devices can be stacked vertically on top of each other, thereby enabling a vertical stack of energy band gaps that using a silicon substrate does not allow. However, as stated before, if the band gap of the junction is engineered and built to capture 1.1 eV energy photons, the junction will efficiently capture the 1.1 eV energy photons but, will not capture lower other energy photons and will waste the photon energy above 1.1 eV. In order to efficiently capture other energy photons, other band gap junctions are built and stacked on the substrate. Typically, these other junctions are tuned for a 1.5 eV energy band gap, a 1.2 eV energy band gap, and a 0.8 eV energy band gap, thereby giving a triple junction device that is capable of capturing photons having greater than 1.5 eV in the 1.5 eV device, and capturing photons having energy between 1.2 eV and 1.5 eV in the 1.2 eV device, and capturing photons having energy between 0.8 eV and 1.2 eV in the 0.8 eV device. However, photons having energy levels of less than 0.8 eV are not captured at all and are wasted and not used. Also, as described earlier, the additional energy of each photon above the energy band gap of the junction in which it is absorbed is also wasted. As an example, a 1.4 eV photon that is absorbed in the 1.2 eV band gap junction loses 0.2 eV of its energy immediately as waste, and only the remaining 1.2 eV of energy is converted efficiently. Another weakness specific to the multiple junction devices is the requirement that current in the device travel in the reverse direction to conventional diode current flow at the transitions between adjacent stacked junctions. This is accomplished by doping the semiconductor material in these transition regions with extremely high concentrations of n-type and p-type elements so that the diode structure becomes a Tunnel diode capable of carrying current in the reverse direction. These tunnel diodes are not perfect conductors of electricity and introduce a loss mechanism specific to multiple junction devices.
Thus, it can be seen that both the conventional diffused silicon p-n junction type of devices and the multi-junction photovoltaic type of devices have fundamental problems in design since they have inherent problems in their basic design it is not possible to capture and convert all the photons that fall on them into free electrons and free holes. Essentially, there are inherent inefficiency problems which stem from the materials that are used to make the devices and the way these devices are engineered and designed.
Both types of photovoltaic devices, the conventional multi-junction photovoltaic cells and the conventional diffused silicon p-n junction type of devices, are more efficient when a concentrated number of photons or an increase in intensity of photons are delivered to the receiving surface of either device type. However, this is also a problem because concentration of photons requires engineering an infrastructure that concentrates photons onto the devices themselves. This infrastructure requires engineering, materials, design, and precision. All of which increases the complexity and the cost of making and implementing photovoltaic devices into the market. Moreover, another problem is that when the photons are obscured, such as with cloud cover or shade, the efficiency is severely diminished if not stopped all together.
Another problem with multi-junction devices is that it is not possible to put an endless number of junctions in the device so as to increase the opportunity to capture and convert photons into useable free electrons and free holes. Moreover, it is not possible to make the photovoltaic device arbitrarily thick. The thickness of the photovoltaic device outside the actual junction has to be approximately one diffusion length, wherein the diffusion length is the approximate length of a path that a charge carrier can travel in a volume of a crystal lattice without an electric field before the charge carrier has a recombination event. The diffusion length generally depends upon the semiconductor material used, the doping of the semiconductor material, and the perfection of the semiconductor material. Generally, there is no totally optimal set condition for all the factors. The conditions are selected on a case by case basis for the particular application. However, it is fair to say that it is a balance of trade-offs, wherein if you reduce the number of free electrons via doping, the resistance increases; if you increase the number of free electrons via doping, the resistance is lowered but the diffusion length is shortened. Thus, in conventional photovoltaic devices no ideal settings of factors can be reached, but rather, a compromise of settings and/or factors is reached which yields a device that does not give the perfect performance, but the factors are compromised so that the best performance is given for the semiconductor material and the environment at hand.
Accordingly, the design and manufacture of conventional photovoltaic devices has several inherent problems all of which have limited conventional photovoltaic devices from achieving their true potential in the market place as well as the expectations of the physics. Because of the fundamental design limitations, fundamental materials limitations, and manufacturing limitations, the costs of manufacturing conventional photovoltaic devices is high. The manufacture, the materials, and the inherent fundamental problems severely limit both the performance of the conventional photovoltaic cell as well as reducing the flexibility of the system architectural design. In order to allow enhanced optimization, design, efficiencies, and performance increases of photovoltaic devices as well as flexibility of architectural design, a new photovoltaic architecture and design is needed.